1. Field of the Invention
The present invention relates to a sphygmomanometer, and in particular relates to a closed type sphygmomanometer.
2. Description of the Related Art
A closed type electronic sphygmomanometer in which a cuff pressing the artery is wound on the upper arm of a patient is already known. For a tonometry hemadynamometry method with this kind of sphygmomanometer, although there are an oscillometric process, a Korotoff's sound method, an impedance method, etc., the oscillometric process has been mainly utilized in clinical applications. A sphygmomanometer in which a tonometry hemadynamometry method utilizing such an oscillometric process is employed is disclosed by, for example, Japanese patent Publication No. 28637 of 1994.
A sphygmomanometer disclosed in the above publication is basically composed of a cuff which is attached to the upper arm of a patient and presses the artery by supplying air therein, a measuring section which is able to detect the superposed pressure of the drop pressure and pulse pressure which change in said cuff and convert the same into digital signals, a digital data processing section which is able to obtain the maximum and minimum blood pressure values of a patient by using the cuff pressure detection signals outputted by said measuring section as input data, and a display section which is able to display the maximum and minimum blood pressure values calculated by said digital data processing section.
With a sphygmomanometer constructed as described above, the maximum and minimum blood pressure values are judged by the digital data processing section on the basis of fluctuations of the pulse pressure vibrations and pulse wave amplitude while causing the pressure in the cuff to drop at a constant rate. However, there were the following shortcomings in this tonometry hemadynamometry method with such an oscillometric process.
That is, in the tonometry hemadynamometry method, as has been represented by an in-artery catheter method, the method for measuring the pressure applied to a blood vessel wall by determining one point of the artery of a patient is ideal. However, with the tonometry hemadynamometry method by the abovementioned oscillometric method, since the pressure fluctuations in the cuff wound on the upper arm of a patient is detected and is used for measuring the blood pressure, a pulse pressure appears even in the in-cuff pressure which is higher than the maximum blood pressure or it is not clear to judge the minimum blood pressure.
With the tonometry hemadynamometry method with a conventional oscillometric process, this results from detecting the mean pulsation of the artery spreading in the range of the cuff, and the pulsation is such that the artery wall displacement of the brachial artery resulting from the heartbeat is propagated as displacements of the skin surface and further the displacement of the skin surface causes the air capacity in the cuff to be changed, wherein this capacity change is detected as a pressure change in the cuff. Resultantly, the displacement quantity of the artery wall is converted to the pressure fluctuation in the cuff.
However, with such a method, since the displacement quantity of the artery wall is measured via air in the cuff, it is not possible to faithfully obtain the artery wall displacement with only the pulse pressure wave obtained from inside the cuff because of receiving influences outside the body such as compression characteristics of air, damping characteristics thereof, etc.
This also means that although constituents of Korotoff's sounds which have higher frequency constituents than the pulse pressure waves are superposed with the pulse waveforms in a cuff pressure dropping process from the maximum blood pressure to the minimum blood pressure in the blood pressure measurement and must appear, fluctuations of higher frequency constituents such as Korotoff's sounds are not able to be propagated since air is used as a propagation medium in the in-cuff pulse pressure waveform in the oscillometric process, and resultantly it seems that such constituents do not appear.
That is, in a tonometry hemadynamometry method with a conventional oscillometric process, since a cuff is wound onto a long length of the brachial artery which is the portion to be measured of a patient and the pulse pressure vibrations are detected as pressure fluctuations in the cuff pressure, the artery pressure of the artery wall at one point which is ideal in the tonometry hemadynamometry method is not accurately reflected. Accordingly, a pulse pressure wave occurs in the cuff pressure which is more than the maximum blood pressure, and since the pulse pressure wave is propagated by using air as a medium, the frequency propagation is adversely influenced by the compression characteristics and damping characteristics of air, and a shortcoming is caused, whereby Korotoff's sound propagation is hindered.